Field of the Invention
The present invention relates to a substrate for mounting integrated circuit semiconductor chips (IC chips), and in particular, the present invention relates to a substrate including accumulated insulation layers made of organic substance.
At the present time, complying with needs of realizing high speeding and small sizing of electronic apparatus, large integration of a semiconductor circuit and miniaturization of a semiconductor device have been advanced as seen in a large scaled integrated circuit semiconductor device (LSI). However, not only the semiconductor device but also a substrate for mounting the semiconductor devices has been advanced. That is, the substrate has been advanced for miniaturizing wiring layers of the substrate, multiplying the wiring layers, decreasing a dielectric constant of an insulation layer in between the wiring layers and, in particular, mounting IC chips of the LSI directly on the substrate. The substrate for directly mounting the IC chips will be called "multichip substrate (MCS)" hereinafter.
In the MCS, an inorganic substance insulation layer, which will be simply called "inorganic insulation layer" hereinafter, made of inorganic substance such as silicon dioxide or alumina has been used as the most popular insulation layer. However, recently, an organic substance insulation layer, which will be called "organic insulation layer" hereinafter, made of organic substance such as polyimide begins to be used. An organic insulation layer made of polyimide, which will be simply called "polyimide layer" hereinafter, is the most typical organic insulation layer. Therefore, the polyimide layer will be discussed hereinafter, representing the organic insulation layer. Because of the general characteristics of organic substance, the polyimide layer is inferior to the inorganic insulation layer with regard to heat-resistance at high temperature. However, the polyimide layer is superior to the inorganic insulation layer with regard to a dielectric constant, flatness and optical sensitivity. The polyimide layer has small dielectric constant such as 3.5, high flatness and optical sensitivity. The optical sensitivity of the polyimide layer is as high as that of photoresist used in fabrication of semiconductor device.
Since the polyimide layer is laid between wiring layers of the MCS, the small dielectric constant of the polyimide layer contributes to decrease parasitic capacitance between the wiring layers. In other words, by virtue of the small dielectric constant of the polyimide layers, it becomes possible to fabricate a thin MCS including multi-wiring layers.
Since a predecessor of polyimide is liquid at room temperature, a very flat thin polyimide layer can be produced by well known spin-coating technique. The polyimide is manufactured by Asahikasei Incorporation (Japanese manufacturer), having a product name "TIMEL" Series G-7600, and 1-Methyl-2-Pyrrolidinon is a solution of the predecessor. By virtue of making the insulation layers very thin and flat, the MCS can be fabricated easily.
By virtue of the optical sensitivity of the polyimide layer, fabrication of multi-insulation layers is easy in comparison with a ease of using the inorganic insulation layers.
As described above, the polyimide layer is very useful for the MCS. However, the polyimide layer has a defect that accumulated multi-polyimide layers are separable when the heat process is carried out in fabrication of the MCS. The separation inherently arises due to water which has oozed out from the polyimide layer and stays on the surface of the polyimide layer in every heat process. This results in accelerating the separation of the accumulated polyimide layers. In particular, the separation occurs when the multi-polyimide layers are accumulated under a substance not passing water. Because, when the multi-polyimide layers are accumulated under the substance not passing water, the water oozed out from the polyimide layers is difficult to be set free to outside of the MCS. This results in lowering stickiness in between the accumulated multi-polyimide layers.
FIG. 1 is a partial cross sectional view of a prior art MCS (100) including multi-polyimide layers (51, 52, 53 and 54) accumulated on a silicon (Si) base substrate (7). FIG. 1 shows that the multi-polyimide layers 51, 52, 53 and 54 are fabricated under a metal pad (PAD) (2) consisting of a part of a wiring layer (34) and a protection film (40) fabricated on the part of the wiring layer 34. The protection film consists of a palladium (Pd) film (41) and a titanium (Ti) film (42). The multi-polyimide layers 51, 52, 53 and 54 are laid between wiring layers (31, 32, 33 and 34) made of aluminum (Al). A part of the wiring layer 34 forms the PAD 2 in a pad region 90. In the pad region 90 or under the PAD 2, no wiring layer lies except (the part of) the wiring layer 34, so that only polyimide layers 51, 52, 53 are accumulated between the PAD 2, which is formed by the part of the wiring layer 34, and a silicon dioxide (SiO.sub.2) (62) insulation layer fabricated on the Si base substrate 7 through another SiO.sub.2 insulation layer. The wiring layers 31, 32, 33 and a wiring layer (35) and another part of the wiring layer 34 are laid in a wiring region 13 with the polyimide layers 51, 52, 53 and 54. The SiO.sub.2 insulation layer 61 is fabricated on the Si base substrate 7 and the SiO.sub.2 insulation layer 62 is fabricated on the SiO.sub.2 insulation layer 61, inserting a wiring layer 35 in the wiring region 13. The SiO.sub.2 insulation layer 62 forms a capacitor between the wiring layers 31 and 35.
In FIG. 1, no wiring layer lies under the PAD 2. This is for avoiding the wiring layers 31, 32 and 33 from being damaged by force added to the PAD 2 when bonding wires, not depicted in FIG. 1, are bonded to the PAD 2.
In FIG. 1, the multi-polyimide layers 51 to 54 are not separated each other. However, since the multi-polyimide layers 51 to 54 are arranged under the PAD 2, there is a fear of arising the separation among the multi-polyimide layers 51 to 54 and between the wiring layer 34 and the polyimide layer 53. FIGS. 2A, 2B and 2C show a case where the polyimide layer 53 is separated from the wiring layer 34 under the PAD 2. FIGS. 2A, 2B and 2C show a part of the cross sectional view of the MCS 100 in FIG. 1, and in FIGS. 2A, 2B and 2C, the same reference numeral as in FIG. 1 designates the same part as in FIG. 1.
FIGS. 2A, 2B and 2C illustrate how the separation occurs in the MCS 100 due to the bonding force. FIG. 2A shows the cross sectional view before the bonding is carried out on the PAD 2. In FIG. 2A, no separation occurs in between the multi-polyimide layers 52, 53 and 54.
FIG. 2B illustrates the cross sectional view when bonding is carrying out by pushing the PAD 2 down with a bonding wire (8) made of gold. The pushing force is shown by an arrow (P) depicted by the bonding wire 8. In FIG. 2B, the PAD 2 is deformed by the pushing force P. However, by virtue of the pushing force P, no separation occurs among the multi-polyimide layers 52, 53 and 54 and between the PAD 2 and the polyimide layers 53.
Generally, a bonding wire is pulled every after bonding has been carried out to a metal pad, for ascertaining whether the bonding is carried out certainly. FIG. 2C illustrates the cross sectional view when the PAD 2 is pulled by the pulling force added through the bonding wire 8. The pulling force is illustrated by an arrow (P') depicted by the bonding wire 8. In FIG. 2C, the PAD 2 (the wiring layer 34) is separated from the polyimide layer 53, producing a gap (9) under the PAD 2. The separation is directly caused by the pulling force P' added to the PAD 2. However, the separation inherently arises due to water which has oozed out from the polyimide layer 53 and stays between the polyimide layer 53 and the PAD 2. The water cannot be set free to outside of the MCS 100 because of the PAD 2 which covers the polyimide layer 53. The water staying between the polyimide layer 53 and the PAD 2 accelerates the separation.
As described above, since the water is hard to be removed from the polyimide layers accumulated under the metal pad, the separation occasionally arises among the polyimide layers or between the wiring layer and the polyimide layer in the heat process of the MCS fabrication. As a result, it has been impossible to raise the production yield rate and the product reliability of the MCS, as far as the organic insulation layers such as the polyimide layers are used in the MCS. This has been a problem of the MCS in the prior art.